Extrusion-Based Additive Manufacturing of WC-10Co Cemented Carbide Produced with Bimodal Ultrafine/Micron WC Particles

Abstract

This article researches the effect of ultrafine (submicron) tungsten carbide powder addition on the microstructure and mechanical properties of WC-10Co cemented carbide produced by the extrusion of a highly filled polymer. This addition aims to develop a material with a good combination of toughness, hardness, and yield strength. The results demonstrate that increasing the ratio between ultrafine and micron WC particles from 0/100 to 45/55 in the initial powder results in successive decreases in average grain size from 2.61 µm to 1.75 µm. When 45% of ultrafine powder is introduced into the mixture, a high number of fine tungsten carbide grains is produced. This promotes inter-grain contact and reduces the free path of the binder phase, which results in a more rigid structure and in the material becoming more brittle. The best mechanical characteristics are achieved in WC-10Co cemented carbide with 15% content of ultrafine powder in the total weight of WC. Here, a microstructure with a bimodal distribution of tungsten carbide grains in a virtually non-intermittent cobalt phase was formed. This allowed us to achieve a compressive strength of 2449 MPa at the deformation of 6.69%, while the modulus of elasticity was 38.8 GPa. The results indicate a good combination of strength and ductility properties in the developed cemented carbide.

1. Introduction

Cemented carbides are composites wherein particles of hard but brittle carbide phases bind with each other, with a soft and ductile metallic binder [1]. Thanks to a unique combination of mechanical, tribological, and physical properties, these materials have found their uses in the manufacturing of cutting, machining, pressing, drilling, and other tools [2,3,4]. A prominent position in the lineup of hard alloys (cemented carbides) is held by composites made of tungsten carbide and cobalt (WC-Co). These play a critical role in the aerospace industry, fossil fuels production, equipment manufacturing, railroad transport, electronic information storage, and other applications [5,6,7,8,9,10]. This composite is mainly produced by liquid-phase sintering when cobalt is used as a binder, as it has almost perfect wettability and has good adhesion to WC grains. This is caused by low free energy at the phase boundary [11]. The main production techniques are based on methods of conventional powder metallurgy [2,5,12], hot isostatic pressing [12,13], and spark plasma sintering [14,15]. These manufacturing processes are rather time-consuming, expensive, and inefficient when producing complex-shape items [16]. Additive manufacturing, an advanced computer-aided manufacturing process, provided a well-fitting solution. This process is used for fabricating items via the layer-by-layer deposition of materials controlled by computer software [16,17,18]. Initially, processes of additive 3D modeling based on partial powder melting using a high-energy radiation source (laser) were used (in so-called powder bed fusion). This hot-forming process includes selective laser sintering, selective laser melting, and selective electron beam melting [16,17,19,20]. Practical research has discovered the main drawbacks of this process when applied to a WC-Co system. The use of a laser causes overheating, leading to microcracks, the anomalous growth of WC grains, the evaporation of Co, and the decarbonation of tungsten carbide WC, with the emergence of unwanted η-phases that are brittle complex carbides, like W₃Co₃C and W₆Co₆C [19,21,22,23]. In the last decade, an additive process based on the extrusion forming of a mixture of metal or composite powders with polymer binder has been finding increasing use [24,25,26]. This process lies in the cold forming of a green body with subsequent debinding and sintering. In the literature, methods such as binder jetting additive manufacturing, fused deposition modeling, and 3D gel printing have been described [27,28,29,30]. This approach is similar to the already-known PIM (powder injection molding) process, based on the injection filling of a blank with a highly viscous powder–polymer solution, which is achieved by applying significant shear force [31,32]. However, this process is mainly used in the large-scale production of small items with complex shapes, where manufacturing requires an ad hoc mold [33]. The main working materials in both processes are feedstocks, i.e., mixtures of a thermoplastic polymer (binder) and powder of metals or composites or both [34]. In the 3D printing process, a 3D item is produced by fused deposition modeling, but melted feedstock is used instead of pure polymer. This causes some key problems: (1) one must ensure that the output of the highly filled polymer melts through the nozzle without the application of shear force, in contrast to the PIM process; (2) one must ensure that the powder fill ratio in the polymer, with sufficient melt ductility, allows for item shape retainment during deposition and debinding, as well as minimizing pore quantity after sintering. Despite these problems and drawbacks, such as complex feedstock preparation and the lengthy process, the material extrusion additive manufacturing (MEAM) process has high potential for the production of various materials from stainless steel, titanium alloys, copper, rare-earth magnets, and ceramics (zirconium, silicon nitride, alumoxide, etc.). This is because MEAM combines the advantages of additive manufacturing and powder metallurgy efficiently enough and hence has been successfully developing in recent years [33,35,36,37]. Materials made of WC-Co alloy have also been produced by using this process [24,25,26,38].

An analysis of the literature has shown that the main problem in the manufacturing of items from cemented carbide lies in achieving high hardness and fracture toughness. It is known that hardness and strength increase as carbide grain size decreases [1,17]. Here, it should be noted that the sintering of WC-Co, whereby a liquid cobalt phase is created, leads to WC grain growth [17,39,40]. The introduction of carbides of transition metals (vanadium, chrome, titanium, etc.) [41,42,43] and compounds of rare-earth elements (yttrium, lantanum, cerium, etc.) is successfully used to prevent anomalous grain growth [41,44,45]. Meanwhile, fracture toughness depends on the content of cobalt binder in the composite while also depending on WC grain size: nanosized and submicron grains cannot deflect fracture propagation from a straight line. It is different for micron grains, which prevent fracture propagation significantly better [46,47]. That is why one of the main objectives lies in the creation of a material microstructure that would allow for both sufficient hardness and high fracture toughness. The common approach to solving this problem consists of using initial powders with bimodal distribution of particles by size, i.e., submicron and micron powders [1,47,48,49]. With this approach, the required distribution of grains by size in the sintered material will be achieved. This opens an opportunity to reach denser WC grain packing (which improves plastic deformation) while also increasing the toughness-to-impact strength ratio by optimizing the free path of the metallic binder phase [1]. This approach has been implemented when fabricating cemented carbides via powder metallurgy and hot isostatic pressing processes [46,49,50,51]. However, this approach has not found its use in the MEAM process for fabricating WC-Co alloys, probably because it significantly increases feedstock viscosity due to inter-grain interactions when ultrafine particles are introduced [52,53,54]. Meanwhile, there are successful examples of using bimodal powders in the MEAM process for creating other materials [55,56].

In this article, an additive method based on the extrusion of highly filled polymer was used to form items with WC-10Co cemented carbide in order to develop a material with a good combination of strength, hardness, and resistance to plastic deformation. For this purpose, mixtures of initial powders with a bimodal particle size distribution were used. In this regard, the purpose of this research is the investigation of the influence of adding ultrafine (submicron) WC powder on the microstructure and mechanical properties of cemented WC-10Co carbides fabricated by an extrusion-based additive manufacturing process.

2. Materials and Methods

2.1. Obtaining Feedstocks

Three-dimensional materials were fabricated by using an extrusion-based additive manufacturing process. Mixtures of commercial-grade powders were prepared for obtaining feedstocks:

−

Tungsten carbide micropowder produced by Kirovgrad hard alloys plant (Kirovograd, Russia) with an average particle size of 6.6 µm (as measured with a Fisher Sub Sieve Sizer), while the total content of carbon was 6.15%, free carbon was 0.03%, and oxygen was 0.013%;

−

Ultrafine tungsten carbide powder produced by Xuzhou Jiechuang New Material Technology Co., Ltd. (Guangzhou, China), with an average particle size of 100–200 nm and a purity of 99.9%;

−

Cobalt nanopowder produced by Xuzhou Jiechuang New Material Technology Co., Ltd. (Guangzhou, China), with an average particle size of 50 nm and a purity of 99.9%.

To adjust WC grain growth during sintering, 0.5 wt.% yttrium oxide was introduced into the mixture (above 100%), specifically YtO-Lum (with a purity of 99.9%) produced by JAC Uralredmet (Verkhnyaya Pyshma, Russia). Dry mixtures were prepared in a Turbula mixer (JSC Vibrotekhnik, St. Petersburg, Russia) over 30 min. The ratios of ultrafine WC powder to micron WC powder were 0/100, 15/85, 30/70, and 45/55. The compositions of the mixtures obtained are listed in

Table 1. Powder mixture compositions used for preparing feedstocks.

Composition No.	Content, wt.%
	WC Micropowder	WC Ultrafine Powder	Co Nanopowder	Y2O3 Powder
1	90	0	10	0.5
2	76.5	13.5	10	0.5
3	63	27	10	0.5
4	49.5	40.5	10	0.5

.

MC 2162 polymer (Emery Oleochemicals, Dusseldorf, Germany), a mixture of polyol and polyamide, was used for preparing feedstocks. The polymer was then mixed with the obtained powder mixtures in a ratio of 50:50 vol.% in a Brabener mixer (Brabener^® GmbH & Co. KG, Duisburg, Germany) at 150 °C for 10 min at 120 rpm. Then, the mixed and homogenized material was extruded through a nozzle with a diameter of 0.8 mm, and the thread obtained was cooled. Then, the thread was cut into 1.5–2.0 mm long pieces that were to be used as granules in 3D printing.

2.2. Extrusion-Based Printing and Post-Processing Process

Extrusion-based 3D printing was carried out on a P3 Steel 300 printer (3DIY, Moscow, Russia). A 20 × 20 × 20 mm box sample with a wall thickness of 3 mm was fabricated with each composition (Figure 1). The printing was performed at constant parameters (

Table 2. WC-10Co box sample printing parameters.

Parameter	Value
Nozzle material	Stainless steel
Nozzle diameter	0.8 mm
Nozzle heating temperature	130 °C
Table heating temperature	60 °C
Printing travel speed	40 mm/s
Filament feed rate (% from the extrusion multiplier)	90%
First-layer height	0.3 mm
Subsequent-layer height	0.2 mm
Infill density	98%
Number of skirts	3
Extrusion multiplier	1.2
Infill pattern	Linear

).

Debinding was performed by the solvent treatment of green bodies in a vessel with acetone over 5 days at room temperature. After the start of sample treatment, the solvent was replaced every 2 days. After debinding, the samples were extracted from acetone, dried, and weighted to determine binder mass loss. After debinding, the samples were sintered in a Nabertherm VHT 8/18-GR retort furnace (Nabertherm, Lilienthal, Germany) at 1440 °C in inert atmosphere. The heating rate was 3 °C/min, and the time of exposure at the sintering temperature was 1 h.

Lastly, 2.5 × ~2.5 × 7 mm (H × W × L) bars were cut out of the sintered boxes by electrical discharged machining flatwise to the printed layers and were to be used as samples for further studying.

2.3. Research Methods

To characterize the rheological properties of the powder-filled polymer (feedstock), the melt flow index (MFI) was calculated by using a Cflow extrusion plastometer (ZwickRoell, Ulm, Germany) with the load at 2.16 kg at the temperature of 150 °C.

The phase composition of the sintered samples was investigated on an XRD-6000 diffractometer (Shimadzu Corporation, Kyoto, Japan) under CuKα radiation. Scanning was performed in the range of angles of 2θ 20–80° at the rate of 2°/min and the scanning pitch of 0.05°. The X-ray diffraction patterns were decoded by using the PDF 4+ database.

The samples’ structure was investigated with a MIRA 3 LMU scanning electron microscope (SEM) (Tescan, Brno, Czech Republic) fitted with an XRF spectrometer to carry out elemental analysis. For further studies, crosswise-polished (i.e., perpendicular to the printing direction) cross-sections of the samples were prepared. The grain size was determined by the linear intercept method by using ImageJ software, version 1.52.

Compression tests were performed on the 2.5 × ~2.5 × 7 mm (H × W × L) bars with an Instron 3369 testing machine (Instron Corporation, High Wycombe, UK). The cross-head speed was 0.2 mm/min.

3. Results and Discussion

3.1. Investigation of Feedstocks and Green Bodies

The WC-Co alloy fabrication process used (additive manufacturing by extrusion of a powder–polymer mixture) envisages not only the investigation into the properties of the powders used as raw materials for fabricating items with cemented carbides but also the investigation of the mixtures (i.e., feedstocks) themselves. For all the compositions, the polymer-to-powder mixture ratio was the same: 50:50 vol.% and 6.2:93.8 wt.%. The numbers of the feedstock compositions and sintered samples corresponded to the powder mixture numbers in Table 1. Increases in ultrafine tungsten carbide particles quantity in the mixture result in increased powder surface area, which need to be wetted with liquid polymer. Accordingly, the finer the particles in the mixture, the thinner the layers of polymer binder between particles. As a result, the viscosity of the filled polymer increases. This was proven by our experiments, where the melt flow index for feedstocks was determined (Figure 2). Already at the first point (13.5 wt.% ultrafine WC in the powder mixture), the MFI decreased by almost three times, from 115.1 g/10 min to 41.2 g/10 min. This trend persisted for further increases in the ratio of ultrafine tungsten carbide particles. It was found that if other parameters remained constant, the output parameter (melt flow index) showed reverse exponential dependency on the input parameter (number of ultrafine WC particles in the mixture), while the correlation coefficient was rather high (R² = 0.9829).

Despite the large area of interaction between particles and polymers at high ratios of replacement of micron powders with ultrafine powders (30 and 45% of ultrafine powder), no lack of polymer was visually observed, and the feedstocks had homogeneous consistency. However, high feedstock viscosity (at 40.5% ultrafine WC content in the powder mixture, the MFI was only 3.2 g/10 min) hinders material extrusion and may lead to the distortion of the shape of the printed green body. At this fill ratio, further increases in the ratio of submicron WC particles would render the 3D printer nozzles unable to extrude feedstock.

The powder mixture without ultrafine WC comprised micron particles (WC particles) and nanosized particles (Co particles). A feedstock based on such composite powder features high fluidity, as nanoparticles can be uniformly distributed around microparticles and act as a solid lubricant (in the so-called ball-bearing effect) [57,58]. The replacement of micron WC particles with submicron particles leads to an increased probability of interaction of same-size particles (probably, submicron WC particles and Co nanoparticles). As a result, densely packed agglomerations may form from particles that can absorb some part of the dispersion phase (i.e., the polymer). Here, the amount of unbound polymer decreases, and the viscosity increases [59,60,61]. Thus, for sufficient feedstock fluidity, the number of microparticles must be optimal at a constant polymer fill ratio in a bimodal powder mixture, while the content of submicron particles and nanoparticles must ensure the “ball-bearing effect” to decrease viscosity. In this regard, the feedstock based on the powder mixture of composition 4 has low potential for extrusion-based additive manufacturing. Composition 3 with 30% of replaced micron WC powder also has rather high viscosity (MFI = 16.2 g/10 min), but it can be used for manufacturing.

As noted above, increasing the content of submicron particles with a uniform distribution of powder and binder in the feedstock leads to a thinner polymer layer between grains. The green body’s properties directly depend on the feedstock structure. In particular, a thinner binder layer results in a decreased removed binder ratio during debinding. This occurred for the investigated compositions 3 and 4, where 30% and 45% of micron WC powder were replaced with ultrafine powder (

Table 3. Shares of binder removed from the green body in the course of debinding in acetone at different contents of ultrafine WC particles in the feedstock.

Indicator	Feedstock Composition No. *
	1	2	3	4
Ratio of ultrafine WC particles to total amount of WC, wt.%	0	15	30	45
Ratio of removed binder, wt.%	91.7 ± 0.5	91.8 ± 0.4	91.1 ± 0.4	90.9 ± 0.5

* Feedstock composition numbers correspond to respective powder mixture numbers in Table 1.

).

The difference between the compositions was not large, under 0.9% in absolute values. The decreased ratio of binder removed may also be proof of the formation of densely packed agglomerates of submicron and nanosized particles that prevented the escape and subsequent removal of polymer binder particles. Accordingly, the composition with 13.5 wt.% ultrafine WC (replacement of 15% of micron WC powder with ultrafine powder) exhibited no such aggregates, or they were too loose or unstable, which allowed for efficient polymer removal, similar to a composition without micron particle replacement.

3.2. Phase and Chemical Composition of Sintered Composites

Sintering is a critical stage in the WC-Co alloys fabrication process. During sintering, the material reaches its final density, and its microstructure is formed.

The X-ray diffraction diagrams of the WC-10Co cemented carbides fabricated by additive manufacturing based on the extrusion of highly filled polymers using powder compositions with different ratios of ultrafine WC after sintering at 1440 °C are shown in Figure 3. The main phases were the hexagonal WC phase (ICDD Card No. 25-1047) and the face-centered cubic (fcc) phase of the Co binder (ICDD Card No. 15-806).

It is evident that the area of this reflex profile at 2θ = 44.3° does not correspond to the amount of Co in the hard alloy (10%). The reason for the low intensity of Co phase reflexes at low Co phase quantities may lie in the possible overlapping of its peaks with the main WC phase [62].

There were no reflexes of other phases, such as graphite or the η-phase (decarbonated phase) that appear when using another 3D printing process implemented by using a high-energy laser [22,63,64]. This is an apparent advantage of extrusion-based printing with highly filled polymer, which has also been noted by other authors [24,25]. This allows for the fabrication of cemented carbides with a homogeneous phase structure without unwanted phases that are detrimental to the mechanical characteristics of the items manufactured [21].

The absence of carbon phase (graphite) reflexes allows for making assertions that the remaining binder is completely removed during sintering. The emergence of residual carbon is possible for the commonly used process of additive extrusion-based printing with feedstocks when single-stage debinding is carried out [38].

An investigation into the elemental composition taken from the cross-section area has shown that all samples generally have a content of carbon that corresponds to the theoretical value of stoichiometric composition of 50 at.% (6.15 wt.% in the initial WC powder and ~5.54 wt.% in the cemented WC-10Co carbide) (Figure 4). The results of the analysis are provided for the composition with 13.5% of ultrafine WC.

The measured cobalt content was lower than that in the initial powder mixture (10 wt.%), which may be attributed to insufficient homogeneity caused by subnormal blending in the powder mixture preparation stage or during feedstock preparation [31]. A similar trend was observed for the other samples, with cobalt content varying in the range 8.07–8.95 wt.%.

3.3. Microstructure of Sintered Samples Depending on Content of Ultrafine WC Particles in Powder Composition

Images of WC-10Co alloys fabricated via the MEAM process with different contents of WC powder are provided in Figure 5. The general structure of the sintered composites can be assessed with small image magnification. In all the samples, except for composition 2, where 15% of WC micropowder had been replaced with ultrafine powder, there were defects represented by pores that were tens of micrometers in size. This principle metallurgic defect in a sintered item is most often related to defects of green body printing [17,24]. For high content of submicron particles, the discovered defects may be attributed to high feedstock viscosity, which caused insufficient mixture homogeneity throughout the printed item bulk (Figure 5c,d). Therefore, excess amounts of fine particles had an influence here. In the case of composition 1 without ultrafine WC particles, the emergence of pores may be attributed to large gaps between grains in the green body that resulted from low poured density of the powder (Figure 5a). The material fabricated with the addition of 15% of ultrafine particles had the most defect-free structure of all (Figure 5b).

It is worth noting that the pores discovered are a result of feedstock inhomogeneity, i.e., their existence is related to the mixture composition and not the fabrication process (3D printing with filled polymer). Here, no defects such as longitudinal pores or fractures or both were observed; those would indicate delamination and ruptures between printed layers.

Several parameters are evaluated to characterize the microstructure of WC-Co cemented carbides: tungsten carbide grain size, number of WC grain contacts, binder volume ratio, and average free path of the binder [1]. These values are interrelated, so WC grain size and Co content are used to describe the microstructure. Figure 6 shows the distribution of tungsten carbide grains by size in the WC-10Co alloys fabricated with different contents of ultrafine WC particles in the raw stock powder. Since the sintering process for the chosen process of the extrusion-based forming of feedstocks is not different from that in conventional powder metallurgy, the cemented carbide grain size will be similar [17]. Accordingly, methods for improving the materials’ structure will be rather similar.

The SEM images of the microstructure in Figure 6 show distinct phase contrast between the components. Light-color carbide grains of prismatic or wedge-like shape were divided by dark cobalt phase binder. The WC grains’ shape corresponds to the one described in other sources [46,49,50,51], including those devoted to the extrusion-based additive manufacturing of WC-Co cemented carbides. In particular, it is noted that truncated triangular prism or polygons with rounded vertexes are typical shapes [24]. It is clear from Figure 6 that irrespective of powder mixture composition, the microstructure of the sintered composite was characterized by a rather homogeneous distribution of WC and Co: the Co binder phase was uniformly distributed around the tungsten carbide grains, forming a continuous facet structure without major agglomerations of the Co phase. Such a structure is optimal for cemented carbides fabricated via different processes, including MEAM [24]. Small pores with a size of up to 5 µm emerged at the edges of rather large WC grains, which resulted in decreased density and hence degraded mechanical properties. In [25], it is noted that this may be caused by excessive sintering temperature. Since the use of a polymer binder in feedstocks may yield residual carbon in the sintered item, higher carbon content causes the eutectic WC-Co temperature to decrease during sintering, so a more liquid phase is produced. This results in larger tungsten carbide grains [25,51]. This is characteristic of cemented carbide composition 4, which had the highest amount of ultrafine WC component characterized by high free surface energy and the highest content of remaining binder after debinding (Table 3). In Figure 7b, we can distinguish that there was certain amount of large ~8–12 µm grains in the structure of this alloy, though a large part of this material had a significantly finer microstructure, as shown in Figure 6. The presence of large grains may have a decisive influence on the values of the mechanical properties of cemented carbide. It should be noted that several large grains in Figure 7b that were outside the measured range (0–9 µm) had virtually no impact on the average grain size.

Still, composition 4 is an exception here. In the sample with 30% of micron powder replaced with ultrafine powder and a similar ratio of polymer binder removed in the course of debinding, such large WC grains were not discovered (Figure 7a). The microstructure here was completely in accord with that in Figure 6. Therefore, we can assert that adding Y₂O₃ powder to the mixture has the intended effect, i.e., it adjusts the dissolution and precipitation processes of the solid phase of tungsten carbide. The effect of rare-earth elements is similar to that of transition metal carbides [41]. The grain growth inhibition effect consists in the formation of thin films of (Y,W)C on WC grains surface, which not only lower interphase energy but also act as a kinetic barrier [1].

Given all the above, it is also evident that changes in the ratio between micron and submicron WC particles in the initial powder affect the microstructure of the sintered composite. We can infer from Figure 6 that as the content of ultrafine WC powder was increased, a gradual narrowing of the distribution caused by a significant reduction in the number of grains larger than 4–5 µm took place. The average grain size decreased gradually: from 2.61 µm at 0% of ultrafine WC to 2.29 µm at 15%, 1.85 µm at 30%, and 1.75 µm at 45%. The cemented carbide particle sizes in all the compositions varied rather widely. According to the Fachverband Pulvermetallurgie classification [65], they combine structures of different materials; however, judging by the average grain size and distribution mode location, they should be classified as medium-sized structures described by grain sizes between 1.4 and 3.4 µm. From the aggregate data by AB Sandvik, materials with a similar grain size and cobalt content combination can be used in mining and construction, including operation under impact loading [1].

Figure 8 shows a comparison of tungsten carbide grain distribution by size. It is fairly clear from the figure that the curve shape does change when ultrafine WC powder is added into the raw stock mixture. As the content of small WC grains increases, the curve gradually shifts to the left, i.e., toward smaller values. This explains a gradual decrease in average particle size.

The phenomenon observed can be explained. The cemented carbides’ sintering process envisages a liquid-phase sintering step. It is accompanied by tungsten carbide grain dissolution in the binder liquid phase until saturation is reached, which is dependent on the ionic product. Finer grains dissolve first. Then, those precipitate into larger grains. Meanwhile, the remaining undissolved coarser grains also grow due to the contribution by the dissolved smaller ones. This mechanism is known as the Ostwald ripening process [66]. It causes an increase in grain size in tungsten carbide and the finishing densification of the structure. The continuous growth of WC grains is inevitable. The driving force of the continuous growth is related to a decrease in the surface energy and interfacial energy of the cemented carbide [41]. However, as the amount of ultrafine WC particles in the initial powder is increased, more finer WC grains remain in the sintered microstructure. This may be attributed to tiny grains that are also deposited from the solution at high contents of submicron fraction of powder and low amounts of micron particles on which initial grain growth takes place [49]. Another explanation suggests that an increase in the amount of ultrafine particles causes increased continuity and adjacency of WC grain boundaries, which results in a greater number of contacts between the grains. Because of this, more opportunities arise for the merger of ultrafine tungsten particles, which yields tiny grains [67]. This phenomenon has also been noted by other researchers [46,49].

The WC grain size distribution histogram for composition 2 had two local peaks at 1.5–2 µm and 4–5 µm, respectively (Figure 6 and Figure 8). Other compositions had predominantly unimodal distribution. The fabrication of a dual-grain structure may improve the mechanical properties of the WC-Co cemented carbide. Small WC grains in WC-Co alloys of such a structure may improve transverse tensile strength and hardness, while coarse grains may promote an increase in fracture toughness by deflecting cracks and trans-granular fractures [47,50]. WC-Co alloys with a bimodal grain structure have a better combination of hardness and impact–abrasive wear resistance compared with materials with a unimodal distribution of WC grains by size. The mechanism of impact–abrasive wear of bimodal solid alloys consists of small grains preventing abrasive wear, while coarse WC grains prevent impact wear [46].

However, it is not only WC grain size that the properties of cemented carbides depend on. The cobalt binder phase plays an important role in the plastic properties of the material, specifically cobalt phase distribution within WC-Co alloy bulk. A large amount of small tungsten carbide grains determines high packing density of the particles in the structure, as well as a large amount of grain contacts. As a result, the size of cobalt bounds is decreased, i.e., the length of free path of the binder phase is reduced, and the structure’s rigidity increases. Here, one of the main mechanical characteristics of the WC-Co alloy, fracture toughness, decreases [51]. This means that the material becomes more brittle. This can probably be applied to the sample based on composition 4, where 45% of micron powder was replaced with ultrafine powder (Figure 9b).

The WC particle packing density of the sintered sample with composition 2 was not so high, and the number of grain contacts was significantly smaller (Figure 9a). As a result, a rather long, continuous binder layer between the WC phase grains was formed, which should ensure good elastic–plastic properties of the material. In this case, crack propagation takes place via the inter-grain mechanism.

3.4. Mechanical Properties

The mechanical properties of WC-10Co cemented carbides were evaluated from uniaxial compression test results conducted on the bar samples. From the compression diagram, we can assess material behavior in the elastic and elastic–plastic deformation stages. As tungsten carbide, i.e., the main component of the WC-Co alloy, is a hard but brittle phase, the compression test evaluated the contribution of the elastic component to the toughness characteristics of cermet as a brittle material.

Stress vs. deformation curves for sintered WC-10Co alloys fabricated via the MEAM process with different contents of WC powder are provided in Figure 10. Material behavior under loading includes two stages: (1) sample deformation in the elastic area, where the loading curve is virtually linear; (2) brittle destruction, where the loading curve has a breakup [55,68].

The curve shape changes in accordance with the material structure formed, which depends on the amount of ultrafine tungsten carbide powder in the initial powder mixture (Figure 10). In the diagrams, not only the final values of stress and deformation at the moment of destruction change, but so does the curve inclination. The lower the inclination angle to the horizontal axis, the more elastic–plastic properties manifest themselves in the cemented carbide. The greater the inclination angle, the more brittle the material is. Given this, the materials can be sorted by increasing the contribution of the elastic component in the following order (in accordance with the initial compositions (the amount of micron powder replaced with ultrafine powder)): 4(45%)→1(0%)→3(30%)→2(15%). Judging by microstructure analysis, the discovered dependency appears to be logically consistent. The sample with composition 4 had a large number of WC grain contacts, which rendered the material brittle. On the other hand, the WC-Co alloy with composition 2 had virtually non-intermittent cobalt binder phase that governed the elastic–plastic behavior of the material (Figure 9a). Such differences in the elastic properties are attributed to the average grain size: materials with finer WC grains are more brittle, whereas materials with coarser grains have higher plasticity [69]. In the present research, a similar dependency is true for alloys with added ultrafine tungsten carbide particles.

The described dependencies are clearly visible in Figure 11. As a ratio of stress to deformation, the calculated modulus of elasticity shows reverse dependence on deformation. The lower the elasticity modulus, the more elastic–plastic properties are exhibited by the material, and vice versa.

Strength is the main mechanical property determined by compression tests. Here, the dependency was similar (Figure 10). The WC-10Co cemented carbide (composition 2) had a strength of 2449 MPa, while the sample with high content of ultrafine particles (45% of micron powder replaced) had the lowest strength of all, 1998 MPa. Such low value can be attributed to the fact that the homogeneous distribution of WC grains in the material could not be achieved because of a large number of grain contacts and because of the formation of coarse tungsten carbide grains (Figure 7b). In the sample with perfect composition (the alloy with 15% of micron powder having been replaced), a more homogeneous dual-grain structure formed, with grain sizes in a rather wide range. This is a result of a uniform distribution of WC particles in the composite bulk under the influence of the liquid Co phase, including the impact of capillary action. However, there were no anomalously coarse grains with size above 10 µm in the alloy (Figure 6). This resulted in a decreased number of defects and pores, while the structure’s density increased (Figure 5).

A similar explanation can be applied to the compression strength vs. average WC grain size curve in Figure 12. It should be noted here that strength as well as hardness should increase as the grain size decreases [17,70]. However, all of this is in good accord with narrow distributions of grain sizes that have mostly one mode. For a rather wide bimodal or polymodal distribution, roles can be divided among all fractions of solid-phase particles in the material structure. The average WC particle size is only an averaged characteristic of a WC-Co cemented carbide, and it does not take into consideration many factors, such as material structure density and the presence of defects and pores. In [49], it is noted that ultrafine WC grains fill interstices between coarse WC grains, decrease the volume ratio of the pores and increase the cemented carbide bulk density, thus ensuring high toughness of the skeleton, which significantly reduces the impact of grain size on hardness.

The compression strength values of the investigated cemented carbides obtained by using the MEAM process surpassed the respective value (998 MPa) of the hard alloy fabricated by using laser powder bed fusion having a similar grain size (2.5 µm) [20]. Results similar to ours were obtained for a WC-Co alloy with added grain growth inhibitors (TaC, NbC, and Y₂O₃) and metallic Ni fabricated via powder metallurgy processes (~2300–3000 MPa) [71]. We could not compare the obtained results with materials additively manufactured via the extrusion-based process, since instead of uniaxial compression tests, cemented carbides are subjected to hardness, fracture toughness, bending, or rupture tests [24,25,26,29]. However, several sources provide common values of ultimate compressive strength for hard alloys, and those are higher than the measured ones (in the range of 3000–9000 MPa [1,2,72]). In some research works, such strength values can be attributed to the submicron size of tungsten carbide grains [73,74]. However, in most cases, the mechanical properties of cemented carbides fabricated by using extrusion-based 3D printing with highly filled polymer are not as high as those manufactured via powder metallurgy process due to low relative density, high porosity, and the presence of cracks [17]. The improvement of the structural characteristics of WC-Co alloys is a key factor for increasing the quality of items manufactured via the MEAM process.

The same can be said about increasing the modulus of elasticity. Values of this parameter found in the literature are significantly higher than those measured herein (in the range of 400–650 GPa [1,5,6,69]). This can be attributed to the material structure with submicron WC grain size, which ensures higher strength (3000–5000 MPa in most cases) and lower deformation (not exceeding 1–2% in most cases) [75]. The deformation of cemented carbides developed for the purpose of this research under compressive uniaxial loading is several times higher, which results in rather low values of the modulus of elasticity. Given all this, the WC-10Co alloys fabricated can be characterized as materials that exhibit rather high plastic properties.

Thus, the best mechanical properties are achieved in the material fabricated from the initial powder mixture with an optimal composition where 15% of micron powder has been replaced with submicron powder. If the feedstock fluidity in the 3D printing stage is high enough, then in the final sintering stage, we can achieve a bimodal material structure with tungsten carbide grains in a rather wide size range, and those are bound by a virtually non-intermittent cobalt phase. The achieved microstructure combination allows for reaching a compressive strength of 2449 MPa at a deformation value of 6.69% and a modulus of elasticity at 38.8 GPa. The results indicate a good combination of strength and ductility properties in the developed cemented carbide.

Before making a conclusion, it should be noted that the mechanical properties of WC-Co obtained by extrusion-based printing with highly filled polymer are somewhat worse than those of composites fabricated via conventional powder metallurgy processes (pressing). However, the MEAM process is an alternative process for manufacturing complex-shape WC-Co items, especially when the requirements of mechanical properties are not so high [25]. The refinement of the manufacturing process and the composition of powder mixture and feedstock are key factors in solving problems related to the improvement of the qualitative indicators of cemented carbides. In regard to the present article, the optimal content of submicron powder in the composition (13.5% of total weight) has turned out to be higher than that reported by other researchers (10 wt.%) [49,51]. Given all this, it can be concluded that the amount of ultrafine particles introduced is individual for each specific alloy being developed, and it depends on the size of micron, submicron, and possibly nanosized powder; the amount of metallic cobalt binder; the mechanism of their mixing and forming; and other process-related factors. All this requires further comprehensive detailed study.

4. Conclusions

The article investigates WC-10Co cemented carbides additively manufactured via the process based on the extrusion of highly filled polymer. The main focus is on the impact of the addition of ultrafine (submicron) WC powder on the feedstock creation process peculiarities and on the microstructure and mechanical properties of the sintered items. The main conclusions are as follows:

• Increasing the content of ultrafine particles in the powder–polymer mixture results in an exponential decrease in the feedstock fluidity indicator. The content of ultrafine WC powder of 15% of total tungsten carbide weight ensures the required rheological properties for 3D printing.
• Changes in the ratio between micron and submicron WC particles in the initial powder affect the microstructure of the sintered composite. Sintered cemented carbide with 15% of ultrafine powder in WC weight features a dual-grain structure with predominant fraction sizes in the ranges of 1.5–2 µm and 4–5 µm. According to literature sources, such a structure improves the mechanical properties of hard alloys. When 45% of ultrafine powder is introduced into the mixture, a high number of fine WC grains promotes inter-grain contact and reduces the free path of the binder phase, which results in a more rigid structure, and the material becomes more brittle. Decreasing the amount of submicron powder allows for the formation of a virtually non-intermittent phase of cobalt binder. This results in improved elastic–plastic properties.
• The best mechanical properties are achieved in a WC-10Co cemented carbide fabricated from the initial powder mixture with optimal composition, where 15% of micron powder had been replaced with submicron powder. The formed microstructure allowed for the achievement of a compressive strength of 2449 MPa at deformation of 6.69%, while the modulus of elasticity was 38.8 GPa. The results indicate a good combination of strength and ductility properties in the developed cemented carbide.